In a medium saucepan, stir together sugar and 3 tablespoons (45 grams) water until moistened. (It should be the consistency of wet sand.) Bring to a boil over high heat, stirring just until sugar dissolves. Using a pastry brush, brush sides of pan with remaining 1 tablespoon (15 grams). (Do not stir once it starts to boil.) Cook, without stirring, until mixture is a dark amber color. Remove from heat; slowly add cream, whisking until combined. Add butter, a few pieces at a time, whisking until melted and well combined. Let cool completely. Refrigerate in an airtight container for up to 1 week. Reheat sauce in microwave in 15-second intervals, stirring between each, until pourable and warm but not hot.